1. Field of the Invention
The present invention relates generally to voltage controlled oscillators used in communication devices and, in particular, to adjusting the frequency of the voltage controlled oscillator by mechanical tuning.
2. Description of the Related Art
RF communication devices, such as microwave transmitters and receivers, require a stable operating frequency provided by an oscillator. Typically, the operating frequency is generated by a voltage controlled oscillator (VCO). The operating frequency of the voltage controlled oscillator is dependent on parts tolerances and manufacturing techniques. A circuit board in the oscillator is used to define a microstrip resonator. Etching variations during manufacturing of the microstrip lines on the circuit board can cause large shifts in the output frequency of the oscillator. Microstrip lines are also used in filters and the same etching variations can cause changes in the filter bandpass or attenuation frequencies
Prior art methods to adjust the operating frequency of VCOs have included using variable capacitors and variable inductors. These methods add cost to the oscillator and have the disadvantage of potentially changing values over time. Another prior art method to adjust the operating frequency of VCOs is to use cutting techniques to remove conductive material from circuit lines This is illustrated by FIG. 1. A conventional microstrip resonator 10 is shown in FIG. 1. Resonator 10 is part of a conventional voltage controlled oscillator circuit It consists of a metal strip 12 on a dielectric substrate or printed circuit board 14. The metal strip 12 is typically etched copper and the circuit board 14 is a typically a laminate of epoxy and fiberglass called FR4. Metal strip 12 has edges 12A and 12B. The microstrip resonator of FIG. 1 is shorted to ground G along edge 12B. The microstrip resonator 10 has an external capacitance C that is part of the oscillator circuit connected to the resonator. A primary cut 20 is shown extending into metal strip 12. The primary cut has ends 20A and 20B. Several cavities 16 are located between primary cut end 20B and end 15 of the metal strip. In between cavities 16 are located shorting lines 18. The length of the primary cut 20 is adjusted by removing additional shorting lines 18 resulting in a longer primary cut. This changes the point of grounding of the resonator and as such changes its equivalent inductance. Every cut increases the effective electrical length and causes a shift in the resonant frequency down or shifts the equivalent circuit inductance higher.
The cutting action can be provided by laser trimming, sand blasting through a mask or mechanically by using a rotating tool. Unfortunately, the technique shown in FIG. 1 has a major drawback. The cutting of additional shorting lines results in large jumps or shifts in frequency of the oscillator with every shorting line 18 that is cut. In some applications, it is required to precisely set the resonant frequency The shorting lines cannot be moved to close to each other because it becomes difficult to accurately cut the shorting lines.
There is a current unmet need for a fine tuning system for VCOs that is permanent and can easily be performed during testing of the VCO. In addition, there is a need for a mechanical fine tuning system for VCOs that causes small changes in operating frequency with a change in the mechanical structure of the circuit board and does not require additional circuit board space.
The present invention provides a fine tuning system for voltage controlled oscillators used in communication devices and, in particular, to adjusting the frequency of the voltage controlled oscillator with fine resolution by mechanical tuning.
The present invention provides a fine tuning apparatus for a resonator. The resonator has a resonant frequency. The apparatus includes a metal strip located on a dielectric substrate. The metal strip has edges. A primary cut extends into the metal strip. The primary cut has a pair of ends. Several cavities in the metal strip are located adjacent an end of the primary cut. An elongated slot is cut into the metal strip. The slot is located between the cavities and an edge. The slot changes the resonant frequency of the resonator in fine increments in proportion to the length of the slot. The fine tuning apparatus does not increase the size of the metal strip.
A further embodiment of the present invention provides a fine tuning apparatus for a resonator. The resonator has a resonant frequency. The apparatus includes a metal strip located on a dielectric substrate. The metal strip has a pair of edges. A primary cut extends into the metal strip. The primary cut has a pair of ends. A first set of cavities in the metal strip is located adjacent an end of the primary cut. A second set of cavities in the metal strip is located between the first set of cavities and an edge of the metal strip. A first set of shorting lines is located between the first set of cavities. The first set of shorting lines, when cut, causes a coarse adjustment to the resonant frequency of the resonator in proportion to the number of the first set of shorting lines cut. A second set of shorting lines is located between the second set of cavities. The second set of shorting lines, when cut, causes a fine adjustment to the resonant frequency of the resonator in proportion to the number of the second set of shorting lines cut. The fine tuning apparatus does not increase the size of the metal strip.